1. Open avast program and add a password to the Avast.Also Check:How to add password to Avast Antivirus
2. After addition of password click on "Additional Protection" on the left pane of avast program.
3. Choose "Site Blocking".
4. On the right pane tick on "Enable site blocking" on the open window.
5. Then on the "URLs to Block" section add the website that you want to block.
6. Click on Enter. (Give the password for confirmation if asked)
7. That is it. Now reboot your system and you will find that the URL are blocked by Avast.